%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-2-22 on page II-142 in "Display environments as mini-pages".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{159.0pt}

%StartShownPreambleCommands
\usepackage{mathtools}
%StopShownPreambleCommands

\begin{document}
\[ \left|
  \begin{multlined}[c]
    (a+b)^2 = \\ (a+b)(a+b) \\ = a^2 + 2ab + b^2
  \end{multlined}                           \right| \]
\end{document}
